Bioinspired approaches to toughen calcium phosphate-based ceramics for bone repair

To respond to the increasing need for bone repair strategies, various types of biomaterials have been developed. Among those, calcium phosphate (CaP) ceramics are promising since they possess a chemical composition similar to that of bones. To be suitable for implants, CaP ceramics need to fulfill a number of biological and mechanical requirements. Fatigue resistance and toughness are two key mechanical properties that are still challenging to obtain in CaP ceramics. This paper thus reviews and discusses current progress in the processing of CaP ceramics with bioinspired microstructures for load-bearing applications. First, methods to obtain CaP ceramics with bioinspired structure at individual lengthscales, namely nano-, micro-, and macroscale are discussed. Then, approaches to attain synergistic contribution of all lengthscales through complex and biomimetic hierarchical structures are reviewed. The processing methods and their design capabilities are presented and the mechanical properties of the materials they can produce are analyzed. Their limitations and challenges are finally discussed to suggest new directions for the fabrication of biomimetic bone implants with satisfactory properties. The paper could help biomedical researchers, materials scientists and engineers join forces to create the next generation of bone implants.